Chronic selenium toxicity results from a sustained daily intake of selenium that exceeds the metabolic rate of selenoprotein turnover.

Study: Selenoprotein P as Biomarker of Selenium Status in Clinical Trials with Therapeutic Dosages of Selenite

When people get a lot more selenium than normal, their bodies keep making more of a protein called SELENOP instead of stopping — which means their bodies can’t keep up with processing all the extra selenium, supporting the idea that too much selenium over time causes toxicity.

Study: Inflammation Mediates the Associations Between Selenium Status and Glucose Metabolism in Chinese Females Over 45 Years Old: a National Cross-Sectional Study.

When people get too much selenium over time, their bodies can't process it all, and it builds up — this study shows that higher levels of a selenium protein in the blood are linked to worse blood sugar and higher diabetes risk, which suggests the body is overwhelmed.

Study: Long-term administration of low-dose selenium nanoparticles with different sizes aggravated atherosclerotic lesions and exhibited toxicity in apolipoprotein E-deficient mice.

When mice got too much selenium over many months — even a small amount every day — their bodies couldn't handle it, and they got sick with damaged organs and more heart disease. This shows that too much selenium over time, not just a big dose once, can be harmful.

Study: Selenoproteins: molecular pathways and physiological roles.

Your body uses selenium to make special proteins that help it work properly. If you eat too much selenium over time and your body can’t use it all fast enough, the extra selenium builds up and can become harmful. This study shows those special proteins are how your body handles selenium, so too much intake can overwhelm them.